Historically, the pneumatic tire has been fabricated as laminated structure of generally toroidal shape having beads, a tread, a belt reinforcement and carcass. The tire is made of rubber, fabric, and steel. The manufacturing technologies employed for the most part involve assembling the many tire components from flat strips or sheets of material. Each component is placed on a building drum and cut to length such that the ends of a component meet, or overlap, creating a splice.
In the first stage of assembly, the carcass would include one or more plies, and a pair of sidewalls, a pair of apexes, an inner liner (for a tubeless tire), a pair of chafers and perhaps a pair of gum shoulder strips. Annular bead cores can be added during the first stage of tire building, and the ply or plies can be turned around the bead cores to form the “ply turnups.”
Typically, the carcass components (excluding the bead cores) would be either “butt spliced” or “lap spliced.” A butt splice has the component ends joined, but not overlapped. A lap splice has overlapping ends.
This intermediate article of manufacture can be cylindrically formed at this point in the first stage of assembly. The cylindrical carcass is expanded into a toroidal shape after completion of the first-stage of tire building. Reinforcing belts and the tread are added to the intermediate article during a second stage of tire manufacture, which can occur using the same building drum or work station or at a separate shaping station.
During the expansion of the carcass, tensile stresses are imposed on the spliced and uncured components of the tire carcass.
In the case of automobile or light truck tires, lap splices were preferred because the splice remained intact, whereas butt splices would tend to open or fail. Even with the good adhesion of the lap splice, the cords adjacent the splice tended to be stretched compensating for the overlapped two layers of cords at the splice. This localized stretching creates a non-uniformity that is readily visible under x-ray, ultrasonic display or by physically cutting the tire and visually inspecting it.
The tire designer, in order to prevent the creation of tire uniformity problems, has historically insured that the splices of various layers of components were not circumferentially aligned. This non-alignment of splice joints was believed to improve the carcass overall durability and uniformity, as measured by the amount of force variation and the balance of the tire. Tire engineers also have believed that tire uniformity could be improved if these discontinuities were deliberately circumferentially spaced around the carcass. This meant that each component had to be applied to the ply at the tire building station where each component was cut and spliced in a spaced order.
When the cord reinforced plies are placed on the building drum, it is very important that the geometric spacing of the beads and the ply turnups are controlled uniformly. Variations in the overall tire building process can result in variations in cord tension. These non-uniformities can affect the ride and handling characteristics of the tire.
In U.S. Pat. No. 6,250,356 to Michelin, a tire assembly drum is disclosed wherein the beads are two distinct sizes. Conventionally, tires are symmetrical having equal bead diameters. The two distinct diameters on a tire exacerbate the problems of tire building and the disclosed assembly drum provides a method and apparatus to permit the tire to be built in a more uniform and faster way. This building drum was designed to build tires having a given set of two different diameters at the first stage of assembly. A separate tire-shaping drum was used to toroidially shape the tire carcass to assemble the tread and belt reinforcements and that drum is disclosed in U.S. Pat. No. 6,234,227.
A conventional prior art approach for the forming of a tire is illustrated in a block level diagram FIG. 7 for the purpose of illustration. In conventional tire forming methods, a drum is collapsed 160 to a D0 diameter and chafer, inner liner, and ply layers are applied. The drum is thereafter expanded 162 to a first diameter D1 and the carcass is completed on the drum by the application of the bead, ply turn up and apply sidewall. The drum is then further expanded 164 to a second diameter D2 and the green tires is thereafter completed on the drum at diameter D2 by the application of breakers and tread components. The drum is then collapsed 166 to its initial diameter D0 and the green tire is removed from the drum. The green tire is loaded 168 into a mold at a press, and a bladder is used to expand the green tire into the mold 168 to a third diameter, D3. The green tire is then cured at the D3 diameter and removed from the mold 170. D3 represents the final diameter of the finished tire.
It will be appreciated that the prior art method of changing the drum diameters from D0 through expanded diameters D1 and D2 in order to arrive at a finished tire diameter D3 acts to exaggerate any imperfections in the tire caused by imperfectly dimensioned tire component layers and/or imperfections caused by uneven drum expansion. The prior art method of FIG. 7 is, to a greater than desired extent, uncontrolled as a result of variables in the carcass layer geometries and the less than perfect control over the expansion of the drum by the press bladder indicated at block 68. Consequently, the prior art methodology may result in a finished tire having less than desired quality and uniformity.